I have heard a lot of people say that the monster transformation on Dark Cloud 2 is pretty much worthless in terms of anything outside of completing the medal acquisition goals for the monster only challenges. For the most part I agree, however, I have found they are very useful in situations where most people would not think to use them especially in boss battles to lay down some serious pawnage.
I recorded this video when I was challenging myself to complete the game with low bare-minimum character stats. You will see that Max and Monica have the lowest amount of hit points and defense points possible.
Basically, I made it all the way up to Gaspard and decided to try a Gemron monster transformation against him with surprising results. So enjoy the footage as I own Gaspard with a Level 5 Gemron. The Gemron monster attacks fast and consistantly enough that I was able to defeat him without getting hit or letting him get a single offensive attack off against me.